Electric Currents and Magnetic Fields

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Electric Currents and Magnetic Fields
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History

• Lodestones were discovered 2000 years ago and
  were magnetic. They were named after Magnesia
  which is a region in Greece where they were
  found.
• The Chinese used them for navigating ships in the
  12th century. We know they contained iron ore
  which is called magnetite.

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Moving Charges Create Magnetic Fields

• In 1820 a Danish scientist Hans Christian Oersted
  noticed that a wire containing current would
  deflect a compass needle.
• This indicated the relationship between
  electricity and magnetism.
• Moving charges create a magnetic field.
• Current running through a wire will create a
  magnetic field.
• The direction of magnetic field can be determined
  by the right hand rule.

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Magnetic field lines

• Bar Magnet Wire with current

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Earths Magnetic Field

• The earth is basically one huge magnet. However,
  the north and south poles are not located exactly
  at the locations we call the north and south
  poles.

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Earths Magnetic Field Changes?

• The angle between the geographic (true) North
  Pole according to the earths magnetic field and
  the magnetic north that compasses point to is
  called magnetic declination.
• Magnetic north has wandered throughout geologic
  time. More than 20 reversals have been recorded
  in the orientation of the dipoles in rocks on
  earth (over 5 billion years!)

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Compasses Align in the direction of the field
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Magnetic fields created by wires
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Charged Particles in Magnetic fields

• Moving charges create a magnetic field.
• When charged particles move through an external
  magnetic field their magnetic fields interact and
  the charge experiences a force.
• The force the charge experiences is always
  perpendicular to the charges velocity and the
  external magnetic field.

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Right Hand Rule

• Use your right hand
• The palm of your hand pushes in the direction of
  the force the charge experiences.
• Your outstretched fingers point in the direction
  of the external magnetic field.
• Your thumb points in the direction of the current
  or the velocity of a negative charge charge
  moving through the external field.

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Right Hand Rule for charges entering a magnetic
field
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Right Hand rule is used to determine the
direction in which the metal rod is deflected
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To calculate the force a charge experiences when
traveling in an external magnetic field

• Fmagnetic Bvqsin?
• Fmagnetic magnetic force exerted on the charge
  by the external magnetic field.
• Unit - Newtons
• q the magnitude of the charge traveling in the
  magnetic field.
• Unit - Coulombs
• v the velocity of the charge traveling in the
  magnetic field
• Unit - m/s
• B The magnitude the the external magnetic field.
• Unit - Tesla
• One Tesla is equal to 1 C of charge is moving 1
  m/s perpendicular to the external magnetic field
  and experiencing 1 N of force.
• sin? angle between v and B

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• Degree of force the charge experiences depends on
  the direction of motion relative to the external
  magnetic field.
• The force has its maximum value when it is
  traveling perpendicular to the magnetic field.
• The force decreases when the charge moves at
  angles to the magnetic field.
• The charge will experience zero force when it is
  moving parallel to the magnetic field lines.
• When a charge is traveling perpendicular to a
  uniform magnetic field the particle will make a
  circular path.

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Right Hand Rule for current Flowing through a
wire.
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Force exerted on a wire carrying charge
perpendicular to an external magnetic field

• Fmagnetic B l I sin?
• Fmagnetic Force wire experiences due to the
  external magnetic field.
• Unit Newtons
• B Magnetic Field
• Unit Tesla
• l (L) Length of the wire
• Unit meters
• I Current in the wire
• Unit amperes
• sin? angle between I and B

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Devices Utilize Electromagnetism

• Electromagnets
• Galvanometer
• DC Motors
• Generators
• Transformers

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Electromagnets

• A coil of wire attached to a battery creates a
  magnetic field inside the coil.
• The overlapping magnetic field lines intensify
  the field inside the coil.
• If a piece of metal is placed inside the coil,
  the domains of the metal become aligned with the
  magnetic field inside the coil and strengthens
  the field.
• Change the direction of current and you change
  the direction of the magnetic field.

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Galvanometers

• Device that measures current using magnetic
  force.
• A coil of wire is wrapped around a soft iron core
  that is mounted on a pivot attached to a needle.
• When current is run through the wire the magnetic
  field created by the current interacts with an
  external magnetic field.
• The degree of deflection is related to the amount
  of current in the coil.

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Electric Motors

• Motors use electricity to create mechanical
  motion.
• Motors use direct current.
• A coil attached to a battery is rotated in the
  presence of a permanent magnet.
• Current flows in opposite directions in the upper
  and lower part of the loop.
• As the current moves through the upper part of
  the loop it is deflected by the permanent
  magnetic field. And rotates half a turn.
• The direction of current is switched every half
  rotation by contacts at the end of the shaft
  which keeps the loop rotating in the same
  direction.

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DC Motor
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DC Motor
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• http//www.walter-fendt.de/ph14e/electricmotor.htm
  

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Electromagnetic Induction

• Electric current can create a magnetic field but
  can a magnetic field create current?
• British scientist Michael Faraday and American
  scientist John Henry in 1831 independently
  discovered the answer is yes.
• They found by moving a magnet in and out of a
  coil of wire electric current is induced.
• A changing magnetic field can create current.
• The amount of current induced is dependent on
• The rate in which the magnet s moved in and out
  of the coli of wire.
• The number of coils in the wire.

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Generator Simulation

• http//www.walter-fendt.de/ph14e/generator_e.htm

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• http//www.physclips.unsw.edu.au/jw/electricmotors
  .htmlmandg